In commonly used brake systems for railway freight trains, each locomotive unit has a control valve that functions to control brake cylinder pressure in response to reductions of brake pipe pressure. This is known as the automatic brake. Each railway freight car also has a control valve that performs a similar function.
There are however differences in the braking requirements of railway freight cars and locomotives which require differences in their brake systems. For effective performance, locomotives are designed with about three times the braking capability of a freight car. This difference produces operating conditions which make it necessary when pulling a train to release an automatic brake application on the locomotive without releasing the car brakes. For example:
(1) When proceeding down a grade during which train speed must be controlled over an extended period of time by brake application, the locomotive wheels would be overheated if its brakes were not released. PA1 (2) In any automatic brake application on a train, the locomotive brake if allowed to apply may cause the cars to "run in" on the locomotive, thereby causing undesirable slack action.
Commonly used locomotive control valves, such as the 26F, 26D and 6NR locomotive control valves, are operative in response to a pressure signal to release the locomotive automatic brake. This is known as "actuating brakes off". The approved ABDW and limited approval ZIAW freight car brake control valves have no need for this feature. On the other hand, the freight car valves include emergency portions which perform functions critical to the rate of transmission and development of brake cylinder pressure in a train in emergency. Emergency functions of this nature are not required of locomotive control valves. Thus, while the freight car and locomotive control valves perform the same basic functions, they are not interchangable because both also perform specialized functions designed to suit their individual service requirements.
Since there are many more railway cars produced and in service than there are locomotives, the freight car brake control valves are produced in much greater volume and at much lower cost than the brake control valves applied to locomotives. Accordingly the cost of a locomotive brake system is proportionally higher than that of a railway freight car.